comments: PDF preview tweaks

* Displayed notes change depending on current previewed PDF page;
* Current page indicator;
* Click on page adds comment on that page;
* Reusable notes macro;
* Fixes notes toggle.

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
comments: refactoring and bug fixing

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>